Photocatalyst antibacterial hydrogel-based fresh-keeping material as well as preparation method and application thereof

Abstract

The invention discloses a photocatalyst antibacterial hydrogel-based fresh-keeping material as well as a preparation method and application thereof, and belongs to the technical field of fresh-keeping materials. The photocatalyst antibacterial hydrogel-based fresh-keeping material is prepared from the following raw materials in parts by mass: 180 to 220 parts of water, 1 to 6 parts of graphite-phase carbon nitride, 10 to 20 parts of polyvinyl alcohol, 1 to 3 parts of carboxymethyl chitosan and 8 to 12 parts of tannic acid. The invention further discloses a preparation method of the photocatalyst antibacterial hydrogel-based fresh-keeping material and application of the photocatalyst antibacterial hydrogel-based fresh-keeping material in fruit fresh keeping. The photocatalyst antibacterial hydrogel-based fresh-keeping material disclosed by the invention solves the problems of potential toxicity, easiness in inactivation, poor fresh-keeping effect and poorer mechanical property of an existing fresh-keeping material, realizes long-time effective fresh keeping of high-moisture fruits, does not involve chemical reaction in the preparation process, is non-toxic, easily available and degradable in raw materials, and has excellent environmental friendliness.

Description

Technical Field

[0001] The invention belongs to the technical field of fresh-keeping materials, and in particular relates to a photocatalyst antibacterial hydrogel-based fresh-keeping material and a preparation method and application thereof. Background Art

[0002] As a major agricultural country, my country produces a large number of different kinds of fruits from south to north, such as strawberries, blueberries, lychees and grapes. However, since most fruits have delicate skins and high water content, they are very easy to rot or dehydrate and brown when left at room temperature. Such problems limit the long-distance transportation and long-term storage of fruits, and thus hinder the expansion of the fruit market, especially for high-value and perishable fruits. At the same time, these fruits consume a lot of resources such as water, soil nutrients and labor during their growth, but due to the difficulty of preserving them, they cannot be transformed into valuable agricultural products in the end. In the long run, this will cause great losses to my country's agricultural production.

[0003] At present, the main methods for preserving fruits include heat treatment, low-temperature storage, controlled atmosphere storage, irradiation treatment and storage with preservatives. Heat treatment can effectively kill microorganisms on the surface of fruits and inactivate the activity of internal enzymes, but heat-treated fruits often have changes in flavor; low-temperature storage can effectively inhibit the growth of microorganisms on the surface of fruits, inhibit respiration and transpiration, but the cost of low-temperature storage is high and it is easy for fruits to suffer from chilling damage; controlled atmosphere storage mainly controls the proportion of gas components in the environment where fruits are located, slows down the metabolism of fruits after picking, and thus prolongs the shelf life, but the cost of controlled atmosphere storage is extremely high and it is difficult for general enterprises to afford it; irradiation treatment is to use radioactive sources to irradiate fruits at low doses to inhibit the growth of microorganisms, but the vitamin content of irradiated fruits will be reduced; preservative storage is to place preservatives around fruits, mainly ethanol, calcium chloride, phytic acid, etc., but some preservatives are easily contaminated on the surface of fruits, and long-term consumption will cause certain harm to the human body.

[0004] Hydrogel-based functional fresh-keeping packaging materials have excellent water retention and suitable gas permeability. In addition, by introducing antibacterial agents, fruits can be effectively protected from microbial erosion and long-term preservation of fruits can be achieved. However, traditional antibacterial agents are potentially toxic and easily inactivated. At the same time, hydrogels have poor mechanical properties and are difficult to meet application requirements. Therefore, the development of a new fresh-keeping packaging material with long-term antibacterial, low price, easy operation and excellent fresh-keeping effect has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the invention

[0005] The problem to be solved by the present invention is to provide a photocatalytic antibacterial hydrogel-based fresh-keeping material and its preparation method and application, so as to solve the problems of potential toxicity, easy inactivation, poor fresh-keeping effect and poor mechanical properties of fresh-keeping materials in the prior art.

[0006] The technical solution adopted to solve the technical problem is to provide a photocatalyst antibacterial hydrogel-based fresh-keeping material, including the following raw materials in parts by weight: 180-220 parts of water, 1-6 parts of graphite phase carbon nitride, 10-20 parts of polyvinyl alcohol, 1-3 parts of carboxymethyl chitosan and 8-12 parts of tannic acid.

[0007] The beneficial effects of the above technical scheme adopted in the present invention are as follows: graphite phase carbon nitride (g-C3N4) is a new type of photocatalyst material with excellent photocatalytic performance. It can effectively decompose organic pollutants and inhibit bacterial growth under light conditions. The introduction of g-C3N4 can improve the mechanical properties of the fresh-keeping material to a certain extent through the nano effect; polyvinyl alcohol (PVA) is water-soluble and has good film-forming and adhesive properties; in the photocatalytic antibacterial hydrogel-based fresh-keeping material, PVA, as the main skeleton material, can form a stable hydrogen bond network with water molecules to enhance the structural stability of the hydrogel; carboxymethyl chitosan (CMCS) is a natural polysaccharide derivative that can interact with the negative charge on the bacterial cell wall through its positive charge to destroy the bacterial cell structure, thereby achieving an antibacterial effect; in addition, carboxymethyl chitosan can also form hydrogen bonds with water molecules to further enhance the moisture retention and stability of the hydrogel. The addition of tannic acid (TA) can endow the preservative material with antibacterial and antioxidant capabilities. It can form a secondary network with CMCS through hydrogen bond cross-linking, and entangle with the primary network of PVA to form a complex cross-linked network, thereby improving the mechanical strength and stability of the hydrogel.

[0008] Preferably, the photocatalytic antibacterial hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 200 parts of water, 2-5 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol, 1-2 parts of carboxymethyl chitosan and 10 parts of tannic acid.

[0009] More preferably, the photocatalytic antibacterial hydrogel-based fresh-keeping material comprises the following raw materials in parts by mass: 200 parts of water, 5 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol, 2 parts of carboxymethyl chitosan and 10 parts of tannic acid.

[0010] The present invention also provides a method for preparing the above-mentioned photocatalyst antibacterial hydrogel-based fresh-keeping material, comprising the following steps:

[0011] (1) dissolving graphite phase carbon nitride in water, stirring, and then ultrasonicating in a water bath to obtain a graphite phase carbon nitride aqueous dispersion;

[0012] (2) adding polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion, heating and stirring to obtain a premixed solution;

[0013] (3) dissolving carboxymethyl chitosan and tannic acid in water and stirring respectively to obtain a carboxymethyl chitosan solution and a tannic acid solution;

[0014] (4) mixing and stirring the premixed solution and the carboxymethyl chitosan solution to obtain a mixed solution;

[0015] (5) sealing and freezing the mixed solution and then thawing it to obtain a hydrogel;

[0016] (6) Soaking the hydrogel in a tannic acid solution to obtain a photocatalytic antibacterial hydrogel-based fresh-keeping material.

[0017] The beneficial effects of the technical scheme adopted in the present invention are as follows: in the present invention, g-C3N4 is first stirred and ultrasonically treated in a water bath to make it uniformly dispersed in water, avoiding the occurrence of agglomeration, and laying a foundation for the subsequent mixing step; polyvinyl alcohol is then added to the graphite phase carbon nitride aqueous dispersion, and the addition of polyvinyl alcohol will enhance the viscosity of the system, help the stable dispersion of g-C3N4 particles in the system, and improve the compatibility between the components; carboxymethyl chitosan solution is then added thereto and sealed and frozen and then thawed, and a primary PVA microcrystal cross-linked network is obtained by a freeze-thaw method through the physical action during the formation and melting of ice crystals, so that CMCS and g-C3N4 molecules are stably dispersed in the hydrogel, and then the hydrogel is immersed in a tannic acid solution, assisted by a secondary functional network formed by cross-linking CMCS-TA, to achieve cross-linking and entanglement between molecules, and obtain a photocatalyst antibacterial hydrogel-based fresh-keeping material with excellent mechanical properties, antibacterial properties, and antioxidant properties.

[0018] Preferably, the stirring in step (1) is stirring at room temperature for 8 to 12 minutes; and the ultrasonic water bath time is 25 to 35 minutes.

[0019] More preferably, the stirring in step (1) is at room temperature for 10 min, and the ultrasonic water bath time is 30 min.

[0020] Preferably, the heating and stirring temperature in step (2) is 90 to 100° C., and the time is 1.5 to 2.5 h.

[0021] More preferably, the heating and stirring temperature in step (2) is 95° C. and the time is 2 h.

[0022] Preferably, in step (3), carboxymethyl chitosan is dissolved in water and stirred for 1.5 to 2.5 hours; tannic acid is dissolved in water and stirred for 0.5 to 1.5 hours; and in step (4), the mixing and stirring time is 0.5 to 1.5 hours.

[0023] More preferably, in step (3), carboxymethyl chitosan is dissolved in water and stirred for 2 hours; tannic acid is dissolved in water and stirred for 1 hour; and in step (4), the mixing and stirring time is 1 hour.

[0024] Preferably, in step (5), the sealed freezing temperature is -10 to -15°C for 3.5 to 4.5 hours; and the thawing is performed at room temperature for 6 to 12 hours.

[0025] More preferably, in step (5), the sealing and freezing temperature is -15°C for 4 hours, and the thawing is performed at room temperature for 6 hours.

[0026] Preferably, the soaking in step (6) is performed at room temperature for 15 to 25 minutes.

[0027] More preferably, the soaking in step (6) is carried out at room temperature for 20 minutes.

[0028] The present invention also provides application of the photocatalyst antibacterial hydrogel-based fresh-keeping material in fruit preservation.

[0029] Preferably, the photocatalyst antibacterial hydrogel-based fresh-keeping material is used in the preservation of strawberries.

[0030] The present invention has the following beneficial effects:

[0031] (1) The photocatalytic antibacterial hydrogel-based fresh-keeping material prepared by the present invention contains two natural polymers, carboxymethyl chitosan and tannic acid, which have antibacterial and antioxidant properties, and realizes light-responsive antibacterial properties by adding g-C3N4. The prepared fresh-keeping material has no toxic side effects and can maintain the moisture of fruits to a greater extent. At the same time, it can avoid microbial erosion of fruits during the preservation process, thereby realizing long-term effective preservation of fruits with delicate skin and high moisture content.

[0032] (2) The photocatalytic antibacterial hydrogel-based fresh-keeping material of the present invention adopts the freeze-thaw method to achieve the crosslinking of PVA microcrystals, and then introduces the hydrogen bond crosslinking network formed by CMCS and TA, and is supplemented with g-C3N4, which effectively improves the mechanical properties and fresh-keeping effect of the hydrogel-based fresh-keeping material; this preparation process does not involve chemical reactions, and only forms the hydrogel-based fresh-keeping packaging material through physical effects, and the raw materials are non-toxic, easily available, and degradable, and have excellent environmental friendliness. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all of the embodiments.

[0034] Therefore, the detailed description of the embodiments of the present invention provided below is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention. The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0035] Example 1

[0036] A photocatalyst antibacterial hydrogel-based fresh-keeping material comprises the following raw materials in parts by mass: 200 parts of water, 2 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol, 1 part of carboxymethyl chitosan and 10 parts of tannic acid.

[0037] In this embodiment, the photocatalytic antibacterial hydrogel-based fresh-keeping material is prepared by the following steps:

[0038] (1) dissolving 2 parts of graphite carbon nitride in 80 parts of water, stirring at room temperature for 10 minutes and then ultrasonically bathing for 30 minutes to obtain a graphite carbon nitride aqueous dispersion;

[0039] (2) adding 15 parts of polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion and heating and stirring at 95° C. for 2 h to obtain a premixed solution;

[0040] (3) dissolving 1 part of carboxymethyl chitosan in 20 parts of water, stirring at room temperature for 2 hours, to obtain a carboxymethyl chitosan solution; dissolving 10 parts of tannic acid in 100 parts of water, stirring at room temperature for 1 hour, to obtain a tannic acid solution;

[0041] (4) mixing the premixed solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0042] (5) The mixed solution was sealed and frozen in a -15°C refrigerator for 4 hours, then taken out and thawed at room temperature for 6 hours to obtain a hydrogel;

[0043] (6) Soaking the hydrogel in a tannic acid solution at room temperature for 20 minutes to obtain a photocatalytic antibacterial hydrogel-based fresh-keeping material.

[0044] Example 2

[0045] A photocatalyst antibacterial hydrogel-based fresh-keeping material comprises the following raw materials in parts by mass: 200 parts of water, 5 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol, 1 part of carboxymethyl chitosan and 10 parts of tannic acid.

[0046] In this embodiment, the photocatalytic antibacterial hydrogel-based fresh-keeping material is prepared by the following steps:

[0047] (1) dissolving 5 parts of graphite carbon nitride in 80 parts of water, stirring at room temperature for 10 minutes and then ultrasonically bathing for 30 minutes to obtain a graphite carbon nitride aqueous dispersion;

[0048] (2) adding 15 parts of polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion and heating and stirring at 95° C. for 2 h to obtain a premixed solution;

[0049] (3) dissolving 1 part of carboxymethyl chitosan in 20 parts of water, stirring at room temperature for 2 hours, to obtain a carboxymethyl chitosan solution; dissolving 10 parts of tannic acid in 100 parts of water, stirring at room temperature for 1 hour, to obtain a tannic acid solution;

[0050] (4) mixing the premixed solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0051] (5) The mixed solution was sealed and frozen in a -15°C refrigerator for 4 hours, then taken out and thawed at room temperature for 6 hours to obtain a hydrogel;

[0052] (6) Soaking the hydrogel in a tannic acid solution at room temperature for 20 minutes to obtain a photocatalytic antibacterial hydrogel-based fresh-keeping material.

[0053] Example 3

[0054] A photocatalyst antibacterial hydrogel-based fresh-keeping material comprises the following raw materials in parts by mass: 200 parts of water, 2 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol, 2 parts of carboxymethyl chitosan and 10 parts of tannic acid.

[0055] In this embodiment, the photocatalytic antibacterial hydrogel-based fresh-keeping material is prepared by the following steps:

[0056] (1) dissolving 2 parts of graphite carbon nitride in 80 parts of water, stirring at room temperature for 10 minutes and then ultrasonically bathing for 30 minutes to obtain a graphite carbon nitride aqueous dispersion;

[0057] (2) adding 15 parts of polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion and heating and stirring at 95° C. for 2 h to obtain a premixed solution;

[0058] (3) dissolving 2 parts of carboxymethyl chitosan in 20 parts of water, stirring at room temperature for 2 hours, to obtain a carboxymethyl chitosan solution; dissolving 10 parts of tannic acid in 100 parts of water, stirring at room temperature for 1 hour, to obtain a tannic acid solution;

[0059] (4) mixing the premixed solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0060] (5) The mixed solution was sealed and frozen in a -15°C refrigerator for 4 hours, then taken out and thawed at room temperature for 6 hours to obtain a hydrogel;

[0061] (6) Soaking the hydrogel in a tannic acid solution at room temperature for 20 minutes to obtain a photocatalytic antibacterial hydrogel-based fresh-keeping material.

[0062] Example 4

[0063] A photocatalyst antibacterial hydrogel-based fresh-keeping material comprises the following raw materials in parts by mass: 200 parts of water, 5 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol, 2 parts of carboxymethyl chitosan and 10 parts of tannic acid.

[0064] In this embodiment, the photocatalytic antibacterial hydrogel-based fresh-keeping material is prepared by the following steps:

[0065] (1) dissolving 5 parts of graphite carbon nitride in 80 parts of water, stirring at room temperature for 10 minutes and then ultrasonically bathing for 30 minutes to obtain a graphite carbon nitride aqueous dispersion;

[0066] (2) adding 15 parts of polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion and heating and stirring at 95° C. for 2 h to obtain a premixed solution;

[0067] (3) dissolving 2 parts of carboxymethyl chitosan in 20 parts of water, stirring at room temperature for 2 hours, to obtain a carboxymethyl chitosan solution; dissolving 10 parts of tannic acid in 100 parts of water, stirring at room temperature for 1 hour, to obtain a tannic acid solution;

[0068] (4) mixing the premixed solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0069] (5) The mixed solution was sealed and frozen in a -15°C refrigerator for 4 hours, then taken out and thawed at room temperature for 6 hours to obtain a hydrogel;

[0070] (6) Soaking the hydrogel in a tannic acid solution at room temperature for 20 minutes to obtain a photocatalytic antibacterial hydrogel-based fresh-keeping material.

[0071] Comparative Example 1

[0072] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 100 parts of water, 15 parts of polyvinyl alcohol and 1 part of carboxymethyl chitosan.

[0073] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0074] (1) dissolving 15 parts of polyvinyl alcohol in 80 parts of water, heating and stirring at 95° C. for 2 hours to obtain a polyvinyl alcohol solution;

[0075] (2) dissolving 1 part of carboxymethyl chitosan in 20 parts of water and stirring at room temperature for 2 hours to obtain a carboxymethyl chitosan solution;

[0076] (3) mixing the polyvinyl alcohol solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0077] (4) The mixed solution was sealed and frozen in a refrigerator at -15°C for 4 hours, and then taken out and thawed at room temperature for 6 hours to obtain a hydrogel-based fresh-keeping material.

[0078] Comparative Example 2

[0079] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 100 parts of water, 15 parts of polyvinyl alcohol and 2 parts of carboxymethyl chitosan.

[0080] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0081] (1) dissolving 15 parts of polyvinyl alcohol in 80 parts of water, heating and stirring at 95° C. for 2 hours to obtain a polyvinyl alcohol solution;

[0082] (2) dissolving 2 parts of carboxymethyl chitosan in 20 parts of water and stirring at room temperature for 2 hours to obtain a carboxymethyl chitosan solution;

[0083] (3) mixing the polyvinyl alcohol solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0084] (4) The mixed solution was sealed and frozen in a refrigerator at -15°C for 4 hours, and then taken out and thawed at room temperature for 6 hours to obtain a hydrogel-based fresh-keeping material.

[0085] Comparative Example 3

[0086] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 200 parts of water, 15 parts of polyvinyl alcohol, 1 part of carboxymethyl chitosan and 10 parts of tannic acid.

[0087] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0088] (1) dissolving 15 parts of polyvinyl alcohol in 80 parts of water, heating and stirring at 95° C. for 2 hours to obtain a polyvinyl alcohol solution;

[0089] (2) dissolving 1 part of carboxymethyl chitosan in 20 parts of water, stirring at room temperature for 2 hours, to obtain a carboxymethyl chitosan solution; dissolving 10 parts of tannic acid in 100 parts of water, stirring at room temperature for 1 hour, to obtain a tannic acid solution;

[0090] (3) mixing the polyvinyl alcohol solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0091] (4) The mixed solution was sealed and frozen in a -15°C refrigerator for 4 hours, then taken out and thawed at room temperature for 6 hours to obtain a hydrogel;

[0092] (5) Soaking the hydrogel in a tannic acid solution at room temperature for 20 minutes to obtain a hydrogel-based fresh-keeping material.

[0093] Comparative Example 4

[0094] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 200 parts of water, 15 parts of polyvinyl alcohol, 2 parts of carboxymethyl chitosan and 10 parts of tannic acid.

[0095] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0096] (1) dissolving 15 parts of polyvinyl alcohol in 80 parts of water, heating and stirring at 95° C. for 2 hours to obtain a polyvinyl alcohol solution;

[0097] (2) dissolving 2 parts of carboxymethyl chitosan in 20 parts of water, stirring at room temperature for 2 hours, to obtain a carboxymethyl chitosan solution; dissolving 10 parts of tannic acid in 100 parts of water, stirring at room temperature for 1 hour, to obtain a tannic acid solution;

[0098] (3) mixing the polyvinyl alcohol solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0099] (4) The mixed solution was sealed and frozen in a -15°C refrigerator for 4 hours, then taken out and thawed at room temperature for 6 hours to obtain a hydrogel;

[0100] (5) Soaking the hydrogel in a tannic acid solution at room temperature for 20 minutes to obtain a hydrogel-based fresh-keeping material.

[0101] Comparative Example 5

[0102] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 100 parts of water, 2 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol and 1 part of carboxymethyl chitosan.

[0103] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0104] (1) dissolving 2 parts of graphite carbon nitride in 80 parts of water, stirring at room temperature for 10 minutes and then ultrasonically bathing for 30 minutes to obtain a graphite carbon nitride aqueous dispersion;

[0105] (2) adding 15 parts of polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion and heating and stirring at 95° C. for 2 h to obtain a premixed solution;

[0106] (3) dissolving 1 part of carboxymethyl chitosan in 20 parts of water and stirring at room temperature for 2 hours to obtain a carboxymethyl chitosan solution;

[0107] (4) mixing the premixed solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0108] (5) The mixed solution was sealed and frozen in a refrigerator at -15°C for 4 hours, and then taken out and thawed at room temperature for 6 hours to obtain a hydrogel-based fresh-keeping material.

[0109] Comparative Example 6

[0110] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 100 parts of water, 5 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol and 1 part of carboxymethyl chitosan.

[0111] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0112] (1) dissolving 5 parts of graphite carbon nitride in 80 parts of water, stirring at room temperature for 10 minutes and then ultrasonically bathing for 30 minutes to obtain a graphite carbon nitride aqueous dispersion;

[0113] (2) adding 15 parts of polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion and heating and stirring at 95° C. for 2 h to obtain a premixed solution;

[0114] (3) dissolving 1 part of carboxymethyl chitosan in 20 parts of water and stirring at room temperature for 2 hours to obtain a carboxymethyl chitosan solution;

[0115] (4) mixing the premixed solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0116] (5) The mixed solution was sealed and frozen in a refrigerator at -15°C for 4 hours, and then taken out and thawed at room temperature for 6 hours to obtain a hydrogel-based fresh-keeping material.

[0117] Comparative Example 7

[0118] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 100 parts of water, 2 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol and 2 parts of carboxymethyl chitosan.

[0119] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0120] (1) dissolving 2 parts of graphite carbon nitride in 80 parts of water, stirring at room temperature for 10 minutes and then ultrasonically bathing for 30 minutes to obtain a graphite carbon nitride aqueous dispersion;

[0121] (2) adding 15 parts of polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion and heating and stirring at 95° C. for 2 h to obtain a premixed solution;

[0122] (3) dissolving 2 parts of carboxymethyl chitosan in 20 parts of water and stirring at room temperature for 2 hours to obtain a carboxymethyl chitosan solution;

[0123] (4) mixing the premixed solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0124] (5) The mixed solution was sealed and frozen in a refrigerator at -15°C for 4 hours, and then taken out and thawed at room temperature for 6 hours to obtain a hydrogel-based fresh-keeping material.

[0125] Comparative Example 8

[0126] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 100 parts of water, 5 parts of graphite-phase carbon nitride, 15 parts of polyvinyl alcohol and 2 parts of carboxymethyl chitosan.

[0127] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0128] (1) dissolving 5 parts of graphite carbon nitride in 80 parts of water, stirring at room temperature for 10 minutes and then ultrasonically bathing for 30 minutes to obtain a graphite carbon nitride aqueous dispersion;

[0129] (2) adding 15 parts of polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion and heating and stirring at 95° C. for 2 h to obtain a premixed solution;

[0130] (3) dissolving 2 parts of carboxymethyl chitosan in 20 parts of water and stirring at room temperature for 2 hours to obtain a carboxymethyl chitosan solution;

[0131] (4) mixing the premixed solution and the carboxymethyl chitosan solution and stirring for 1 h to obtain a mixed solution;

[0132] (5) The mixed solution was sealed and frozen in a refrigerator at -15°C for 4 hours, and then taken out and thawed at room temperature for 6 hours to obtain a hydrogel-based fresh-keeping material.

[0133] Comparative Example 9

[0134] A hydrogel-based fresh-keeping material comprises the following raw materials in parts by weight: 80 parts of water and 15 parts of polyvinyl alcohol.

[0135] The hydrogel-based fresh-keeping material in this comparative example is prepared by the following steps:

[0136] (1) adding 15 parts of polyvinyl alcohol to 80 parts of water, heating and stirring at 95° C. for 2 hours to obtain a polyvinyl alcohol solution;

[0137] (2) The polyvinyl alcohol solution was sealed and frozen in a refrigerator at -15°C for 4 hours, then taken out and thawed at room temperature for 6 hours to obtain a hydrogel-based fresh-keeping material.

[0138] Experimental example

[0139] 1. The mechanical properties of the fresh-keeping materials prepared in Examples 1 to 4 and Comparative Examples 1 to 9 were tested. The testing method is as follows:

[0140] Test steps: Refer to "Determination of tensile properties of plastics Part 3": Test conditions for films and sheets to test mechanical properties. Cut the fresh-keeping materials prepared in Examples 1 to 4 and Comparative Examples 1 to 9 into 3mm*3mm*30mm strips, and use a universal testing machine equipped with a 200N sensor to test the mechanical properties. The tensile rate is 5mm / min, and the tensile strength is measured. The tensile strength calculation formula is:

[0141] σ=F / (a×b)

[0142] Where F represents the load (N); a represents the width of the specimen (mm); b represents the thickness of the specimen (mm); and σ represents the tensile strength (kPa).

[0143] The elongation at break is calculated as follows:

[0144]

[0145] Where l0 represents the initial length of the sample (mm), and l represents the length of the sample after stretching (mm).

[0146] Young's modulus is an indicator of material stiffness, which is obtained by linear fitting slope of stress-strain curve between 10% and 50% strain, and toughness is obtained by the area enclosed by stress-strain curve and X-axis. According to Lake-Thomas model, for general engineering materials, the higher the stiffness, the worse the toughness, and the better the toughness, the worse the stiffness.

[0147] The test results of mechanical properties of the fresh-keeping materials prepared in Examples 1 to 4 and Comparative Examples 1 to 9 are shown in Table 1.

[0148] Table 1 Test results of mechanical properties of fresh-keeping materials

[0149]

[0150]

[0151] By comparing the tensile strength of Comparative Examples 1 to 2 with Comparative Example 9, Comparative Example 9 is a PVA microcrystalline single network hydrogel. After adding CMCS (Comparative Example 1), a PVA / CMCS semi-interpenetrating network hydrogel is formed, and its mechanical properties are significantly improved from 92 kPa to 258 kPa, the Young's modulus is increased to 3.9 kPa, and the toughness is increased to 2438 kJ m -3 ; By comparing Comparative Examples 1-2 with Comparative Examples 3-4, after soaking in TA solution, a large number of hydrogen bonds are formed between TA and CMCS, forming a PVA / CMCS-TA double network hydrogel with the PVA microcrystalline network, and the tensile strength is further increased to 416 kPa, the Young's modulus is increased to 7.9 kPa, and the toughness is increased to 5746 kJm -3; By comparing Comparative Examples 1 to 2 with Comparative Examples 5 to 8, after adding g-C3N4, the tensile strength, Young's modulus and toughness are all improved. This is because hydrogen bonds are formed between g-C3N4 and the molecular chain, and the hydrogel is effectively enhanced by the nano effect; By comparing Examples 1 to 4 with the comparative examples, the photocatalyst antibacterial hydrogel-based fresh-keeping material prepared by the present invention has a tensile strength of up to 749 kPa, a Young's modulus of up to 18.4 kPa, and a toughness of up to 8016 kJ m -3 The construction of double network hydrogel is supplemented by nano g-C3N4, and its mechanical performance index is significantly better than that of the hydrogel-based fresh-keeping material prepared in the comparative example. Its mechanical properties can be greatly improved, providing a strong guarantee for subsequent fresh-keeping.

[0152] 2. The fresh-keeping effects of the fresh-keeping materials obtained in Examples 1 to 4 and Comparative Examples 1 to 9 were evaluated. The specific operation and testing methods are as follows:

[0153] Strawberries were used as test objects. The fresh-keeping materials prepared in the above-mentioned Examples 1 to 4 and Comparative Examples 1 to 9 were used to coat the surfaces of the strawberries respectively. Strawberries not coated with any fresh-keeping materials were used as blank controls. The strawberries were placed at room temperature for 5 days to test the weight loss rate, peel hardness, and titratable acid content of the strawberries, and to evaluate their fresh-keeping effects.

[0154] 2.1 Method for determining the weight loss rate of strawberries:

[0155] The weight loss rate of strawberries was determined by weighing method, and the calculation formula was:

[0156]

[0157] Where m0 represents the initial mass of fresh strawberries (g); m t represents the weight of the strawberry after 5 days (g); η represents the weight loss rate (%).

[0158] 2.2 Method for determination of strawberry hardness

[0159] The hardness of strawberries was measured using a handheld hardness tester (GY-2 model). Three points at the same distance from the equator of each strawberry were tested; the arithmetic mean of the results was taken as the effective hardness value.

[0160] 2.3 Determination of titratable acid content in strawberries

[0161] The titratable acid content was determined according to the acid-base neutralization principle. First, the strawberry pulp was ground and transferred to a 50 mL volumetric flask to make up the volume. After standing at room temperature for 30 minutes, it was filtered. 20.0 mL of the filtrate was transferred, 2 drops of phenolphthalein (1%) were added, and the solution was titrated with a standardized NaOH solution until the solution turned red and faded after 30 seconds. The measurement was repeated twice. The titratable acid content was calculated as follows:

[0162]

[0163] Wherein, V0 represents the volume of strawberry solution (mL); N represents the concentration of NaOH solution (0.1 mol / L); 0.064 is the conversion coefficient of citric acid (g / mmol); V1 represents the consumed volume of NaOH solution (mL), m represents the sample mass (g), and V2 represents the titrated strawberry filtrate volume (mL).

[0164] The test results of strawberry weight loss rate, hardness and titratable acid content obtained according to the above formula are shown in Table 2.

[0165] Table 2 Test results of fresh-keeping effect of fresh-keeping materials

[0166] project Strawberry weight loss rate (%) Strawberry hardness (kPa) Strawberry titratable acid (%) Example 1 8.5 188 68.8 Example 2 9.4 192 70.6 Example 3 5.9 332 89.1 Example 4 3.8 343 90.5 Comparative Example 1 25.7 158 53.7 Comparative Example 2 19.3 267 73.5 Comparative Example 3 10.8 160 57.9 Comparative Example 4 4.5 227 81.2 Comparative Example 5 20.6 164 59.3 Comparative Example 6 21.3 193 61.6 Comparative Example 7 20.4 292 75.9 Comparative Example 8 22.7 305 80.4 Comparative Example 9 48.9 32 30.3 Blank control 59.2 30 27.3

[0167] As can be seen from Table 2, the weight loss rate of strawberries not packaged with hydrogel (blank control) after 5 days of storage at room temperature reached 59.2%, the strawberry fruit was almost completely softened, the fruit hardness was only 30 kPa, the titratable acid content dropped to 27.3%, and a large amount of its nutrients were lost; while the weight loss rate of strawberries coated with PVA microcrystalline single network hydrogel (Comparative Example 9) dropped to 48.9%, and the hardness and titratable acid content were both improved, indicating that the hydrogel has a significant effect on the water retention of strawberry fruit; By comparing Comparative Examples 1 to 2 with Comparative Example 9, it can be seen that after adding CMCS, the hydrogel has a certain antibacterial property, avoids microbial erosion, the fruit hardness is increased to 267 kPa, the weight loss rate is as low as 19.3%, and the titratable acid content is as high as 73.5%, indicating that the introduction of CMCS can significantly improve the preservation performance of hydrogel fresh-keeping packaging materials for strawberries. By comparing comparative examples 1 to 2 with comparative examples 3 to 4, it can be seen that after the antibacterial and antioxidant components TA are continuously introduced into the system, the preservation effect of the hydrogel fresh-keeping packaging material on strawberries is further improved. After being placed at room temperature for 5 days, its weight loss rate is as low as 4.9%, the fruit hardness is as high as 227 kPa, and the titratable acid content reaches 81.2%. By comparing and analyzing the effects of the embodiments and the comparative examples, it can be seen that the preservation effect of the photocatalyst antibacterial hydrogel-based fresh-keeping material prepared by the present invention on strawberries is significantly better than the preservation effect of the comparative example fresh-keeping material on strawberries. The weight loss rate of the strawberries after 5 days of preservation of the photocatalyst antibacterial hydrogel-based fresh-keeping material prepared by the present invention is as low as 3.8%, the fruit hardness reaches 343 kPa, and the titratable acid content is as high as 90.5%, which has a good preservation effect.

[0168] The present invention has been described according to the above embodiments. It should be understood that the above embodiments do not limit the present invention in any form. Any technical solutions obtained by equivalent replacement or equivalent transformation shall fall within the scope of the present invention.

Claims

1. A photocatalyst antibacterial hydrogel-based fresh-keeping material, characterized in that: The invention comprises the following raw materials in parts by weight: 180-220 parts of water, 1-6 parts of graphite carbon nitride, 10-20 parts of polyvinyl alcohol, 1-3 parts of carboxymethyl chitosan and 8-12 parts of tannic acid.

2. The photocatalyst antibacterial hydrogel-based fresh-keeping material according to claim 1, characterized in that: The invention comprises the following raw materials in parts by weight: 200 parts of water, 2-5 parts of graphite carbon nitride, 15 parts of polyvinyl alcohol, 1-2 parts of carboxymethyl chitosan and 10 parts of tannic acid.

3. The photocatalyst antibacterial hydrogel-based fresh-keeping material according to claim 2, characterized in that: The method comprises the following raw materials in parts by weight: 200 parts of water, 5 parts of graphite carbon nitride, 15 parts of polyvinyl alcohol, 2 parts of carboxymethyl chitosan and 10 parts of tannic acid.

4. The method for preparing the photocatalytic antibacterial hydrogel-based fresh-keeping material according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) dissolving graphite phase carbon nitride in water, stirring, and then ultrasonicating in a water bath to obtain a graphite phase carbon nitride aqueous dispersion; (2) adding polyvinyl alcohol to the graphite phase carbon nitride aqueous dispersion, heating and stirring to obtain a premixed solution; (3) dissolving carboxymethyl chitosan and tannic acid in water and stirring respectively to obtain a carboxymethyl chitosan solution and a tannic acid solution; (4) mixing and stirring the premixed solution and the carboxymethyl chitosan solution to obtain a mixed solution; (5) sealing and freezing the mixed solution and then thawing it to obtain a hydrogel; (6) Soaking the hydrogel in a tannic acid solution to obtain a photocatalytic antibacterial hydrogel-based fresh-keeping material.

5. The method for preparing the photocatalytic antibacterial hydrogel-based fresh-keeping material according to claim 4, characterized in that: The stirring in step (1) is performed at room temperature for 8 to 12 minutes; and the ultrasonic water bath time is 25 to 35 minutes.

6. The method for preparing the photocatalytic antibacterial hydrogel-based fresh-keeping material according to claim 4, characterized in that: The heating and stirring temperature in step (2) is 90-100° C. and the time is 1.5-2.5 h.

7. The method for preparing the photocatalytic antibacterial hydrogel-based fresh-keeping material according to claim 4, characterized in that: In the step (3), carboxymethyl chitosan is dissolved in water and stirred for 1.5 to 2.5 hours; the tannic acid is dissolved in water and stirred for 0.5 to 1.5 hours; and in the step (4), the mixing and stirring time is 0.5 to 1.5 hours.

8. The method for preparing the photocatalytic antibacterial hydrogel-based fresh-keeping material according to claim 4, characterized in that: In the step (5), the sealing and freezing temperature is -10 to -15°C for 3.5 to 4.5 hours; and the thawing is performed at room temperature for 6 to 12 hours.

9. The method for preparing the photocatalytic antibacterial hydrogel-based fresh-keeping material according to claim 4, characterized in that: The soaking in step (6) is performed at room temperature for 15 to 25 minutes.

10. Use of the photocatalytic antibacterial hydrogel-based fresh-keeping material according to any one of claims 1 to 3 in fruit preservation.

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